In the face of increasingly stringent emission requirements, engines have been required to increase the level of treatment of exhaust for compliance. Diesel particulates and various undesired nitrogen-oxygen compounds collectively referred to as “NOx” are treated using diesel particulate filters and selective catalytic reduction (SCR) units, respectively.
SCR units use a reductant such as urea in the presence of a catalyst to react NOx into less objectionable compounds such as nitrogen (N2), water (H2O) and carbon dioxide (CO2). The amount of urea to inject into the SCR is a function of NOx measured at the output of the engine. Readings of NOx taken after the SCR unit are used for a number of things including closed-loop DEF dosing control for high NOx conversion efficiency and for detection of tampering.